The pleasure of reading history is that it's so much more unexpected than the news. Who would have thought that one of the great philosophers of the enlightenment defended Oliver Cromwell's massacres in Ireland? Or that he did so in ways that are commonly used to justify our own actions today?

We don't now argue about Cromwell as if he were a contemporary. When he slaughtered thousands at Wexford and Drogheda, this seems to us now an act of inexcusable barbarity; perhaps the kind of thing that only happens in religious wars – and the English civil war was certainly religious.

Not so David Hume. In his history of Britain he justifies this as excusable barbarity: by slaughtering without quarter all of the civilian inhabitants of the first towns to resist his armies, Cromwell pacified the rest of Ireland without needless bloodshed:

"Though twice repulsed with loss, [Cromwell] renewed the attack, and himself, along with Ireton, led on his men. All opposition was overborne by the furious valour of the troops. The town was taken sword in hand; and orders being issued to give no quarter, a cruel slaughter was made of the garrison. Even a few, who were saved by the soldiers, satiated with blood, were next day miserably butchered by orders from the general. One person alone of the garrison escaped to be a messenger of this universal havoc and destruction. His justice was only a barbarous policy, in order to terrify all other garrisons from resistance. His policy, however, had the desired effect."

Nor was Hume expressing anti-Catholic prejudice here. For when General Monck did the same in his campaign through Hume's native Scotland, the philosopher was once more detached:

"Dundee was a town well fortified, supplied with a good garrison under Lumisden, and full of all the rich furniture, the plate, and money of the kingdom, which had been sent thither as to a place of safety. Monk appeared before it; and having made a breach, gave a general assault. He carried the town; and following the example and instructions of Cromwell, put all the inhabitants to the sword, in order to strike a general terror into the kingdom."

I don't think any modern historian would dare express himself quite so brutally on this question. Certainly, Hume was much less shocked and condemnatory about these war crimes than he was by Cromwell's religious fervour and hypocrisy. Some people would argue that this shows we have grown more sensitive, more civilised, than in the early days of the enlightenment. I doubt that. Here's why:

The argument that massacring civilians now saves us from having to massacre more of them in future is absolutely central to the end of the second world war. The justification for bombing Hiroshima and Nagasaki was exactly that hundreds of thousands of deaths immediately were morally better than millions later. I know that some of the allies were not much concerned about Japanese deaths – only about their own – but some among them were concerned about all the casualties of war, on either side.

I happen to believe this calculation was correct. I also think that the related calculations of the cold war, about deterrence, were morally correct. They did avert the use of nuclear weapons, and that was the right thing to do. Of course, you need far fewer weapons than the world now has for a deterrent to work, and to deter, but that's another argument. What matters is that to threaten convincingly great civilian suffering can sometimes be correct; and you can't make a convincing threat unless you are prepared to carry it out.

But I have to admit this argument makes me uneasy. This is partly because the lesser of two evils is still very much an evil. It is also because it makes the justification of an action consequent on its success, and that cannot be foreseen. Suppose your act of calculated terror fails to frighten the enemy into submission. What then? More terror? This is clearly a case in which a mistake is worse than a crime, or adds to it.

Yet the alternative, absolutist position which holds you should never do certain things, however good the consequences may be, seems an evasion of responsibility. I do in fact take an absolutist position on torture, and certain other human rights. I'm much less sure it is generally applicable.

Questions of this sort are one reason why the doctrine of Original Sin makes sense to me. It is an attempt to account for the indisputable fact that we live in a universe where some moral choices are both unavoidable and more or less all bad. It may not be a successful attempt – and in any case, it's not an answer, but a way of thinking about the problem – but it is better than denying the reality it tries to explain.